The present invention generally relates to superalloy structures subject to excessive wear, such as components of gas turbines and other turbomachinery. More particularly, this invention relates to a method of repairing worn surfaces of a gas turbine bucket formed of nickel-base superalloys that are prone to cracking when welded.
Superalloys are used in the manufacture of components that must operate at high temperatures, such as buckets, nozzles, combustors, and transition pieces of industrial gas turbines. During the operation of such components under strenuous high temperature conditions, various types of damage or deterioration can occur. For example, wear and cracks tend to develop on the angel wings of latter stage buckets as a result of rubbing contact between adjacent nozzles and buckets. Because the cost of components formed from superalloys is relatively high, it is typically more desirable to repair these components than to replace them. For the same reason, new-make components that require repair due to manufacturing flaws are also preferably repaired instead of being scrapped.
Methods for repairing nickel-base superalloys have included gas tungsten arc welding (GTAW) techniques. GTAW is known as a high heat input process that can produce a heat-affected zone (HAZ) in the base metal and cracking in the weld metal. A filler is typically used in GTAW repairs, with the choice of filler material typically being a ductile filler or a filler whose chemistry matches the base metal. An advantage of using a ductile filler is the reduced tendency for cracking. An example of weld repair with a ductile filler is the use of IN617 and IN625 superalloys to repair worn angel wings of buckets cast from IN738 and equiaxed nickel-base superalloys such as GTD-111. A significant advantage of using a filler whose chemistry matches the base metal is the ability to more nearly maintain the desired properties of the superalloy base material. An example of this approach is weld repairing GTD-111 superalloy buckets with weld wires formed of GTD-111 or René 80 superalloy. To reduce the likelihood of cracking, the base metal typically must be preheated to a high temperature, e.g., about 700 to 930° C. With either approach, the GTAW process can distort the base metal due to the build up of high residual stresses. Components with complex geometries, such as buckets of gas turbines, are less tolerant of distortion, to the extent that GTAW may not be a suitable repair method, particularly if a ductile filler cannot be used.
More advanced directionally-solidified (DS) nickel-base superalloys are often not as readily weldable as the GTD-111 superalloy, further increasing the risk of cracking in the weld metal and within the HAZ of the base metal. A notable example is the nickel-base superalloy GTD-444, which is finding use for latter stage (e.g., second or third stage) buckets in advanced industrial gas turbines due to its desirable creep resistance properties. GTD-444 is not readily weldable primarily due to its higher gamma prime (y′) content (about 55 to 59%), and previous attempts to weld it have produced unacceptable cracking in the base metal HAZ and weld metal.
In view of the above, alternative repair methods are required to repair high gamma-prime nickel-base superalloys that will yield crack-free repairs. For repairing the wear-prone surfaces of such superalloys, it is also necessary that the repair material also exhibit excellent wear properties. One such approach is termed activated diffusion healing (ADH), examples of which are disclosed in commonly-assigned U.S. Pat. Nos. 5,902,421 and 6,530,971. The ADH process employs an alloy powder or mixtures of powders that will melt at a lower temperature than the superalloy component to be repaired. If two powders are combined, one of the powders is formulated to melt at a much lower temperature than the other powder, such that upon melting a two-phase mixture is formed. Vacuum brazing causes the braze powder mixture to melt and alloy together and with the superalloy of the component being repaired. A post-braze diffusion heat treatment cycle is then performed to promote further interdiffusion, which raises the remelt temperature of the braze mixture.
Another alternative repair approach disclosed in commonly-assigned U.S. Pat. No. 6,398,103 to Hasz et al., involves brazing a wear-resistant foil to a worn surface of a component. The foil is formed by thermal spraying a wear-resistant material on a support sheet. Suitable wear-resistant materials include chromium carbide materials and Co—Mo—Cr—Si alloys, such as the commercially-available TRIBALOY® T400 and T800 alloys. Still another approach disclosed in commonly-assigned U.S. patent application Ser. No. 10/708,205 involves the use of a braze tape formed by firing a pliable sheet containing powders of a braze material and a wear-resistant alloy in a binder. The tape is applied to the repair surface, after which a heat treatment is performed to cause the braze tape to diffusion bond to the repair surface so as to define a built-up surface, which can then be machined to the desired dimensions for the repair.
With the advent of more highly alloyed superalloys, improved repair methods and materials are required that are specialized for the particular surface being repaired, including the superalloy and the strength and microstructure required by the repair. A notable example is the need for materials and processes tailored to perform repairs on components with complex geometries and formed of superalloys having high gamma-prime contents, such as GTD-444.